Patent Literature 1 discloses an apparatus that resonates light from a plurality of semiconductor lasers with an external resonator and combines the beams into one beam. In the invention disclosed in Patent Literature 1, a polarizing beam splitter and a retardation plate are provided for each set of a plurality of semiconductor lasers and one emission mirror is used to configure an external resonator, and the retardation plate is adjusted so as to combine emission beams into a polarized and coherently combined high energy beam.